What are the manifestations of rich and lean mixture ratios?

[Elle]

Phenomena of excessively rich or lean air-fuel mixture are as follows: 1. Lean mixture symptoms: Carburetor backfire, difficult starting, engine overheating, detonation in cylinders, power loss (these symptoms improve when choke is closed), knocking sound during acceleration, unstable idle speed prone to stalling, very white spark plugs. 2. Rich mixture symptoms: Black smoke exhaust with possible oil droplets, engine overheating, insufficient power, unstable operation, increased fuel consumption, muffled engine sound, weak acceleration, unstable idle speed, severely blackened and carbon-fouled spark plugs. 3. Rich mixture: When the excess air coefficient is between 0.85-0.95, flame propagation speed reaches maximum, enabling fastest combustion that achieves peak cylinder pressure and temperature in shortest time with minimal heat loss, producing maximum work output. Since slightly more fuel is supplied than required for complete combustion at fixed air volume, oxygen utilization improves, increasing combustion product molecules and gas pressure - thus enabling the engine to deliver maximum power. This mixture is called the maximum power mixture.